I think PetSmart and PetCo and other large chain stores churn them out faster and younger because of their push to sell small tanks and cute "baby" fish. Mom and pop stores sell older ones, at least here they do. I bet Lakitu and Ludendorff (both from the same lps) were about a yr old. They idk, look older. Everytime I look at PetSmart bettas they seem younger. In size and appearance. Bowser, from PS, was smaller at purchase than most at the lps.
I always thought about age too. It could also be smaller fish from stores like PetSmart are like.. inbred or something in the water makes them smaller. There's a lot of emphasis here on the forum about separating them and cleaning the water so it doesn't inhibit their growth patterns. Maybe the dirty tanks don't start at the petstore level.